1. Field of the Invention
This invention relates to an electroluminescence device comprising a luminescence layer comprising (i) an organic compound having positive-hole-transporting property and (ii) a fluorescent organic compound, capable of directly converting electric energy to photo-energy under application of a voltage thereto, and providing a light emitting device having a luminescent plane surface larger than those of an incandescent lamp, a fluorescent lamp or a light emitting diode.
2. Discussion of Background
Electroluminescence devices are classified into two types depending on the exciting mechanism for the emission: (1) an intrinsic electroluminescence device, in which electrons and positive holes migrate locally in the luminescence layer to excite a luminescent material contained therein, capable of emitting light only in an alternating electric field, and (2) a carrier injection type electroluminescence device, in which electrons and positive holes are injected by electrodes and recombined with each other in the luminescence layer to excite a luminescent material, capable of emitting light only in a direct electric field.
In general, the intrinsic electroluminescence device contains a luminescent material made of an inorganic compound such as ZnS to which Mn or Cu is added. The electroluminescence device of this type has the drawback that a high alternating electromotive force of 200 V or more is required for the operation thereof. In addition, this device is disadvantageous in that it requires high production cost, emits light with low luminous intensity, and has low durability.
On the other hand, the carrier injection type electro-luminescence device has a luminescence layer made of a thin layer of an organic compound and can emit light with high luminous intensity. For instance, electroluminescence devices which emit green light are disclosed in Japanese Laid-Open Patent Application 59-194393 and U.S. Pat. No. 4,720,432, and an electroluminescence device which emits yellow light is reported in Jpn. Journal of Applied Physics, Vol. 27, Pages 713-715. One or both sides of the luminescence layer of each of the above electroluminescence devices, there is provided with a positive hole injection layer or an electron injection layer. These electro-luminescence devices can emit light with high luminous intensity in a direct electric field even when a low electromotive force of 100 V or less is applied thereto.
However, the carrier injection type electroluminescence device is composed of multiple thin layers made of organic compounds, and these layers are required to be as thin as 1000 .ANG. to 3000 .ANG., uniform and free from pin holes. Therefore, not all organic compounds are usable for the electroluminescence device of this type. In addition, the production process of the carrier injection type electro-luminescence device is complicated because the multiple layers of the organic compounds are formed by vacuum deposition.
Furthermore, a carrier injection type electroluminescence device which can emit light with high luminous intensity and is highly durable has not been obtained yet. In particular, there is no electroluminescence device capable of emitting blue light as yet.